Materials to fabricate a high resolution plasma display back panel

ABSTRACT

Materials for making a plasma display having a transparent front panel spaced from a back panel which is a metal core having layers of a dielectric material extending over and bonded to the core. The materials of which the back panel is made are chosen to form a back panel having a thermal coefficient of expansion compatible with that of the front panel. The dielectric material is made from a green ceramic tape which is bonded to the core and cofired with the core to form the back panel. The materials for the dielectric can be chosen such that the composite TCE of the cofired assembly matches the TCE of the front panel.

This application claims the priority of U.S. Provisional Application No.60/201,341 filed May 2, 2000 entitled MATERIALS TO FABRICATIONRESOLUTION LTCC-M BACK PANEL.

FIELD OF THE INVENTION

The present invention relates to materials for making a plasma displaydevice and method of making the same, and, more particularly, tomaterials for making a plasma display in which the back panel is made ofa metal core having layers of a dielectric material thereon and metalelectrodes on and between the dielectric layers.

BACKGROUND OF THE INVENTION

A typical plasma display includes a front panel and a back panel bothmade of sheet glass (e.g. conventional float-glass). Electricalconnections and mechanical structures are formed on one of both of thepanels. For example, the back panel may have a ribbed structure formedon it such that the space between the ribs defines a pixel in a directcurrent (DC) display or column of pixels in an alternating current (AC)display. The ribs prevent optical cross-talk, that is to say, plasmafrom one pixel leaking into an adjacent pixel. Fabrication of theseribbed structures, called barrier ribs, poses a challenge both in thematerials and manufacturing techniques that are used.

Plasma displays operate by selectively exciting an array of glowdischarges in a confined rarefied noble gas. Full color displays aremade by generating a glow discharge in a mixture of gases, such as He—Xeor Ne—Xe gas mixture to produce ultraviolet light. The ultraviolet lightexcites phosphors in the pixel cell, as defined by the barrier ribs, toproduce light of desired color at the pixel position.

A typical plasma display back panel comprises a glass substrate having aplurality of substantially parallel, spaced first electrodes on asurface thereof. In AC displays, a thin layer of a dielectric material,such as a glass, covers the electrodes. Barrier ribs are formed on thesurface of the glass substrate between the first electrodes. The barrierribs project from the surface of the substrate at a distance greaterthan the thickness of the first electrodes. Red, green and blue (R-G-B)phosphor layers overlie alternating columns of the first electrodes inthe spaces between the barriers and also overlie the walls of thebarriers. A front transparent glass substrate, the front panel, overliesthe rear panel and may rest on the barrier ribs so as to be spaced fromthe rear glass substrate by the barrier ribs.

Typically, the barrier ribs are walls which define troughs or channelson the back panel. Alternating current (AC) plasma displays typicallyhave barriers that form the separators for the column pixels, and hence,have continuous vertical ribs on the back plate. By contrast, directcurrent (DC) plasma displays typically have ribbed barriers whichisolate each pixel from all of its neighbors. Thus, for DC displays, therib structure has a rectangular lattice-like layout. In either case, thedesired resolution for the display device and its size determine thesize of the ribbed barriers. In a typical display, the ribs are 0.1 to0.2 mm in height, 0.03 to 0.2 mm wide and on a 0.1 to 1.0 mm pitch.

The barriers may be formed on the back plate by laminating a ceramicgreen tape to the back plate, sandblasting the green tape to form thechannels between the barriers and then firing the back plate in a kilnto convert the green tape barriers into ceramic barriers, as set forthin U.S. Pat. No. 6,140,767, entitled “PLAMSA DISPLAY HAVING SPECIFICSUBSTRATE AND BARRIER RIBS” to Sreeram et al.

The front panel includes an array of substantially parallel, spacedsecond electrodes on its inner surface. These second electrodes extendsubstantially orthogonally to the first electrodes. A layer of adielectric material, typically glass, covers the second electrodes. Alayer of MgO covers the dielectric layer. Voltages applied to theelectrodes in the proper manner excite, maintain and extinguish a plasmain the gas within the region formed by the barriers. Addressing ofindividual pixels is done using external circuitry at the periphery ofthe panel. Barrier structures are typically used to confine thedischarge to the addressed pixel, eliminating both electrical andoptical cross talk between adjacent pixel elements. The columns ofpixels are separated by the barriers, and the first electrodes arearranged beneath the gaps between the barriers. In a DC plasma display,the electrodes are not covered with glass or MgO, and the barrierstructures are typically crossed, providing a box-like structure at eachpixel element.

SUMMARY OF THE INVENTION

A composition of materials to fabricate high resolution, low-temperaturecofired ceramic-on-metal plasma display back panels, comprising a metalsubstrate having a predetermined thermal coefficient of expansion and aglass material formulation bonded to the metal core, comprising at leasttwo glasses such that the thermal coefficient of expansion of theformulation closely matches that of the metal substrate.

In an exemplary embodiment of the invention, the metal substrate is madeof titanium.

In another exemplary embodiment of the invention, the glass materialformulation is comprised of at least two glasses. The first glassmaterial includes ZnO MgO, B₂O₃, and SiO₂, and has a formulation, bypercent weight, of:

ZnO up to 30 MgO up to 25 B₂O₃ up to 20 SiO₂  up to 25;

and a second glass material has a formulation defined by weight percentas:

BaO 65.20-87.20 B₂O₃  2.60 -16.10 SiO₂   9.10-35.00.

In yet another exemplary embodiment of the invention, the mixture of thefirst and second glass materials has a thermal coefficient of expansion(TCE) nearly equal to that of the titanium metal substrate. A glassmixture with a TCE nearly equal to that of the titanium metal substratemay have a formulation defined by percent weight as:

first glass 81.1-65.00 second glass  10.9-35.00.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing the method of making the back panel ofthe present invention;

FIG. 2 is a flow chart showing the method of making the green tapes usedin making the back panel of the display of the present invention;

FIG. 3 is an isometric drawing of an embossing tool suitable for use informing barrier ribs for a plasma display according to the presentinvention.

FIG. 4A is a cut away side plan view of an embossed barrier ribaccording to the present invention.

FIGS. 4B and 4C are cut away side plan views which are useful fordescribing the formation of a barrier rib structure which is thicker atthe top than at the bottom.

FIG. 5 is a flow chart showing the method of making a plasma displaypanel of the present invention;

FIG. 6A is a cut-away side plan view of a first plasma display panelaccording to the present invention.

FIG. 6B is a cut-away side plan view of a second plasma display panelaccording to the present invention.

DETAILED DESCRIPTION

To improve the processing of plasma displays, a type of plasma displayhas been developed wherein the back panel comprises a core plate of ametal and layers of a dielectric material extending over, and bonded tothe core plate. On the surfaces of the dielectric layers and between thedielectric layers are metal strips forming the various electrodes. Thisback panel is made by forming thin green tape layers of a dielectricmaterial and coating the surfaces of at least some of the green tapelayers with metal strips or conductive ink which form the electrodes.The green tape layers are placed on the surface of the core plate andthe assembly is fired at a temperature at which the green tape layersfuse together and bond to the core plate. The firing operation is donein ambient air; no specialty gasses are needed.

A plasma display according to the present invention includes a frontpanel made from glass, such as a float glass which has a thermalexpansion coefficient of about

8.5×10⁻⁶/° C., it is desirable that the back panel have a thermalcoefficient of expansion compatible with that of the front panel. Thisprevents disrupting the seal between the back panel and the front panelduring the operation of the display. For a back panel which comprises ametal core plate having layers of a ceramic material bonded thereto,novel materials and a novel materials processing system are used toensure that the back panel is compatible with the glass front panel.FIG. 1 shows the various steps used to form a back panel according toone embodiment of the present invention.

Briefly, the components of the back panel are a metal core 110, ceramicgreen tapes 112, conductor inks 114 and phosphor inks 116. As describedbelow, the metal core 110 of the back panel may be formed from metallictitanium. The ceramic green tape 112 is made as described below withreference to FIG. 2. The preparation of the conductor inks is alsodescribed below. The phosphor inks may be any of a number of commonlyavailable preparations, such as those used to apply phosphors to cathoderay tubes (CRTs).

The first step in the process, step 118, prints the electrodes and fillsvias on the green tapes using the conductor inks and an optionalconductive fill paste, the composition of which is described below. Theelectrodes and conductive vias may be implemented on several layers ofthe green tape, as described below, and interconnected using viasthrough the green tape layers. These electrodes and vias allow theceramic structure formed from the fired green tape to include theelectrical connections between the pixel cells and the drivingelectronics, mounted, for example, along the edges of the display.

The next step in the process, step 120, is to stack and laminate thevarious green tape layers. In the exemplary embodiment of the invention,because vias are formed through at least some of the green tape layers,it is desirable to precisely align the various green tape layers beforethey are laminated. In the exemplary embodiment of the invention, theceramic green tape may be laminated using a pressure of approximately 40Kg/cm² and a lamination temperature of approximately 90° C.

After the green tape has been laminated, the metal core is prepared, atstep 122, by spraying or screen printing a bonding compound onto thesurface which is to receive the green tape. As described below, for theexemplary embodiment of the invention, this compound may be powderedGlass 1, a component of the exemplary ceramic green tape. Also at step122, the green tape is attached to the metallic core and is embossed orscribed to form the barrier ribs. Although, in the exemplary embodimentof the invention, the barrier ribs are formed after the green tape isattached to the metallic core, it is contemplated that the green tapemay be processed to form barrier ribs before it is attached to the core.At step 124, the combination of the metallic core and shaped green tapeare co-fired at a peak temperature of approximately 900° C. After theco-fired back panel has cooled, the phosphor stripes are printed betweenthe column barrier ribs and the back panel is baked to affix thephosphors. The application of the phosphors and the baking used to affixthem may be any of a number of conventional processes commonly used toaffix phosphors, for example, to CRT screens.

As described above, it is desirable for the thermal coefficient ofexpansion (TCE) of the metal core plate to be matched to that of thefront panel. It is desirable to use soda-lime float glass (TCE about8.5×10⁻⁶/° C.) for the front panel because of its low cost. A metal withthis TCE, or other arbitrary value, can generally be synthesized bylaminating of different metals, e.g., Cu—Mo—Cu. In this laminate, theouter metal has a TCE that is different from the TCE of the inner metal,and the laminate takes on a TCE value intermediate between the two. Theexact value depends on the relative thickness and other properties ofthe different layers. However, it is more convenient to chose anexisting metal or alloy that meets the TCE requirement. One exemplarymaterial is metallic titanium, which has a TCE of 8.5×10⁻⁶/° C. Titaniumis a rugged material, with the highest strength to weight ratio of anymetal or metal alloy. Titanium is an abundant metal, readily availablein vary large sheet form, moreover, it is relatively inexpensive.

Together with the metal core, the ceramic green tape is a chiefinventory component of the back panel. As shown in FIG. 2, green tapefabrication beings with inorganic raw materials, such as MgO, Al₂O₃,SiO₂, B₂O₃, P₂O₅, PbO, ZnO, TiO₂ and various alkali, alkaline or heavymetal oxides or materials formed from them. The ingredients are mixed ina batch in proportions to achieve the desired properties. This batch ismelted at 1400°-1700° C., and is quenched. The resulting glass is groundto form a powder. The glass powder is combined with organic binders,solvent, surfactants, and other modifier additives to form a slurry. Theslurry is spread on a flat surface, for example, by a doctor bladeprocess. The process of forming large sheets of ceramic tape is referredto as “casting”. The cast tape formed by this process, including theglass powder, is easily stored in rolls.

The ingredients that constitute the tape, both the inorganic oxides andthe various organics, are selected to provide desired tape properties.For a back panel formation, these tape properties include a TCE veryclosely matched with the TCE of the metal substrate, the ability to beformed in large area casting, the ability to be embossed or scribed toform high-resolution barrier ribs and the ability to maintain barriershape during firing. Furthermore, the tape materials are desirably fullydensified at a peak firing temperature of the furnace as low as 750° C.Ceramic green tapes are commonly cast in sizes of one to two meters wideand several meters in length. For such large area casting, the organicbinders may be formulated to provide high tear strength for handlingduring manufacturing. It is also desirable to ensure uniform tapethickness and homogeneity throughout the cast. The organic componentsmay also be selected to promote uniform lamination at nominal pressures(10-10,000 Kg/cm²).

The barrier ribs may be formed on the back panel by a single embossingstep while the ceramic tape is in the green state, i.e., prior tofiring. Proper embossing depends on a combination of plastic andvisco-elastic flow properties of the laminate. These flow properties arecontrolled principally by the organic resins blended into the slurryused to prepare the tape, and on the particle size and distribution ofthe inorganic ceramic powders used in the slurry.

During firing, the organics that promote the barrier formation arequickly burned off. The remaining ceramic powders melt and crystallize.The temperature of melting and of crystallization and the rate ofcrystallization vary from material to material. The ceramic tapecomposition described below provides desired melting and crystallizationproperties while also providing desirable properties in the finalceramic layers, including a thermal coefficient of expansion whichmatches that of the metallic core.

As the ceramic materials in the tape melt during the firing process,they tend to flow. Crystallization impedes this visco-elastic flow, andpromotes solidification. To maintain the barrier shapes embossed intothe green tape, the ceramic powder combination desirably has acrystallization temperature just lightly above the softening point(750-850° C. for the green tape composition described below withreference to Table 1). This allows the material to become more dense atthe softening point, and to flow sufficiently to develop a smoothsurface. The rapid crystallization, however, causes the glass tomaintain the shape and form of the barriers that were fabricated whenthe tape was embossed or scribed in its green state.

The thermal coefficient of expansion (TCE) of the final ceramic can becontrolled by combining glasses with TCEs above and below the desiredvalue. The ratio of these constituent glasses is adjusted to obtain thedesired TCE. Control of TCE is important to minimize stresses in thefinal panel and assure panel flatness after cooling. A formulation whichproduces a ceramic tape having a thermal coefficient of expansionmatching that of the titanium back panel and having the other desiredproperties for a ceramic tape is shown in Table 1 as weight percentranges.

TABLE 1 Glass 1 25.00-55.00  Glass 2 0.00-15.00 Glass 3 0.00-10.00 Resin0.00-35.00 TiO₂ 0.00-10.00 ZrO₂ 0.00-10.00 Zn stearate 0.00-2.00  50/50isopropanol 0.00-10.00 methyl ethyl ketone

Glass 1 is a zinc magnesium borosilicate glass which is ground to apowder using a standard comminution process. The formulation of Glass 1is shown in weight percent formulation in Table 2:

TABLE 2 ZnO up to 30 MgO up to 25 B₂O₃ up to 20 SiO₂ up to 25

Glass 2 is a barium silicate glass with boron trioxide and is ground toa powder using a standard comminution process. The formulation of Glass2 by weight percent is shown in Table 3:

TABLE 3 Barium Oxide (BaO) 65.20-87.00  Boron Oxide (B₂O₃) 2.60-16.10Silicon Dioxide (SiO₂) 9.10-23.00

Although the compositions given in Table 1 perform well, it has beenshown by the inventors that compositions corresponding to the ranges,defined by percent weight, set forth in Table 4, produce ceramicmaterials with a TCE very close to that of the titanium back panel, andare acceptable for use with the present invention.

TABLE 4 ASAP Formulation: 123 131 149 162 183 206 229a 229 235a 229b229c 229d Component: Glass 1 (3.5-10 um) 52.29 51.28 47.62 46.24 41.1643.80 45.43 46.17 46.62 31.63 33.70 32.12 Glass 2 (3.5-4.5 um) 13.0712.82 11.90 11.56 10.29 3.61 5.54 5.63 5.68 7.92 8.44 8.04 Resin 2 16.3416.03 16.37 16.76 15.44 13.69 14.20 14.43 14.57 20.30 21.63 14.02 Resin3 18.30 17.95 18.15 17.92 Resin 1 21.35 22.36 24.73 23.57 23.80 25.7920.94 31.25 TiO2 (0.5-1.5u) 1.92 1.73 0.95 4.40 4.47 3.61 6.29 6.71 6.39white spinel (3.5-4.5u) 5.95 5.78 5.15 3.53 ZrO2 (1.5-3.5u) 5.15 3.534.57 4.65 3.76 6.54 6.97 6.64 Zn stearate 0.48 0.57 0.51 0.51 0.71 0.760.72 L7602 0.51 0.55 0.57 0.58 0.58 0.81 0.87 0.82 MSR-80 (4.5-5.5u)8.46 Cordierite (3.5-4.5u) 0.87 ASAP Formulation: 229e 229f 230 231 232233 234 235 236 237 238 238a Component: Glass 1 (3.5-10 um) 34.69 47.8628.12 29.40 30.75 31.78 28.88 29.20 29.19 30.63 29.21 30.47 Glass 2(3.5-4.5 um) 8.68 11.98 7.04 7.36 7.03 7.28 7.23 7.31 7.31 7.67 7.317.63 Resin 2 20.49 12.29 18.41 18.87 19.00 20.11 18.54 18.74 18.74 19.6618.75 19.56 Resin 3 Resin 1 20.40 32.13 30.94 31.04 32.85 32.30 32.6532.64 34.25 32.66 29.86 TiO2 (0.5-1.5u) 6.90 9.52 6.32 5.85 5.28 6.154.65 5.23 3.05 4.65 4.85 white spinel (3.5-4.5u) ZrO2 (1.5-3.5u) 7.179.89 6.53 6.08 5.47 6.48 5.41 4.83 5.40 3.17 4.83 5.04 Zn stearate 0.781.08 0.72 0.75 0.67 0.70 0.74 0.75 0.75 0.79 0.75 0.68 L7602 0.89 1.230.72 0.75 0.76 0.80 0.74 0.75 0.75 0.79 0.75 0.78 MSR-80 (4.5-5.5u)Cordierite (3.5-4.5u) 1.12 50/50 IPA/MEK 6.14 Fused SiO2 (1.4u) 1.08Geltech SiO2 (1u) 1.12 ASAP Formulation: 239 240 240a 241 242 243 244245 229d 229e 246 247 Component: Glass 1 (3.5-10 um) 31.18 29.58 31.1829.00 23.88 30.88 34.63 32.59 34.31 34.69 31.32 31.41 Glass 2 (3.5-4.5um) 7.80 7.40 7.81 7.26 5.98 7.73 8.15 8.59 8.68 7.84 7.86 Resin 2 19.3018.99 22.02 20.47 20.27 19.82 18.82 19.71 14.97 20.49 20.11 20.16 Resin3 Resin 1 31.53 31.01 25.42 30.40 30.95 30.21 30.74 29.96 30.64 30.73TiO2 (0.5-1.5u) 4.29 3.83 4.04 4.33 5.03 3.07 6.96 3.57 6.90 1.87 1.88white spinel (3.5-4.5u) ZrO2 (1.5-3.5u) 4.45 3.98 4.04 4.50 5.22 3.197.34 3.71 7.09 7.17 1.94 1.95 Zn stearate 0.68 0.66 0.70 0.65 0.71 0.690.75 0.69 0.77 0.78 0.70 0.71 L7602 0.77 0.76 0.80 0.74 0.81 0.79 0.750.79 0.88 0.89 0.80 0.81 MSR-80 (4.5-5.5u) Cordierite (3.5-4.5u) 50/50IPA/MEK Fused SiO2 (1.4u) Geltech SiO2 (1u) 1.16 0.83 1.10 1.93 PASglass (12.3u) 3.79 4.00 2.65 F-92 (0.75u) 5.98 Panel glass asahi (2.9u)3.62 3.67 2.57 ESOL-2 33.38 20.40

The formulation for exemplary Resins 1, 2, and 3 are given in Tables 5,6, 7, and 8 below, respectively.

TABLE 5 Resin 1 methyl ethyl ketone 30.00 g Ethanol 30.00 g #160plasticizer (Monsanto) 20.00 g B-98 resin (SPI) 20.00 g 100.00 g 

TABLE 6 Resin 2 Methyl ethyl ketone 30.03 g Ethyl alcohol 30.03 g Fishoil 39.94 g 100.00 g 

TABLE 7 Resin 3 methyl ethyl ketone 36.05 g Ethanol 36.05 g #160plasticizer 11.16 g B-98 resin 16.74 g 100.00 g 

The process for forming the ceramic green tape is shown in FIG. 2. Thisprocess begins with the raw materials. As described above Glass 1 and,optionally, Glass 2, are separately prepared by mixing (step 214) abatch, melting (step 216) the batch at the temperatures and for thetimes described above, quenching (step 218) the molten mixture to form aglass and grinding the glass to form a powder. In the exemplaryembodiment of the invention, the glasses Glass 1 and Glass 2 are groundto have a particle size of approximately 10 microns. The fillers,Forsterite and, optionally, Cordierite are ground to have a particlesize of approximately 6.5 microns.

Next, at step 222, the ground glasses and fillers are combined in a 250ml beaker with the organics in the proportions shown in Table 4 to forma slurry. For this step, the glass powder mixture is thoroughly mixedfirst with Solvent 1 and then with Resin 1. The slurry mixture is thenmilled, also in step 222, by placing the slurry in a one liter millingjar containing 150 ml ⅜″ZrO₂ rollers and placing the jar on a roll millfor at least two hours. The slurry is then strained into a container toremove the rollers and de-aired for one minute while stirring.Incomplete de-airing results in pinholes or small bubbles in the casttape. Excessive de-airing results in the removal of too much of thesolvent, which results in a slurry which is too viscous.

This slurry is formed into sheets at step 224 using a doctor bladeprocess. The first step in this process is to measure the viscosity ofthe de-aired slurry. A typical viscosity is between 700 and 1100 cps.Next, a 3-mil mylar sheet is prepared by applying a silicon releaseagent to the top surface of the sheet. Next, the doctor blade isadjusted to obtain the desired thickness of the tape. A blade opening of15 mils is used to obtain a green tape thickness of 8 mils. The slurryis then poured into the tape caster. Initially, the mylar is pulled at aspeed of 20 cm/min until the slurry appears at the output area of thetape caster. Then the pull speed is increased to 90 cm/min. Because ofthe abrasive properties of the slurry, the doctor blade and the tapecasting head may periodically need to be reground or replaced. At step226, the sheets are dried for at least 30 minutes before being removedfrom the tape caster. As a quality control step, the cast tape is viewedon a light box for imperfections and the thickness and density of thetape are measured at several points along the cast tape. At step 228,the green tape is punched to prepare blanks which may have conductor inkand fill paste applied to them to form electrodes and interconnectingcircuitry, at step 118 of FIG. 1, before being laminated, at step 120,to the metal core.

An exemplary composition of a conductor ink is set forth in Tables 8 and9 and the composition of a suitable fill paste is set forth in Table 10.

TABLE 8 Conductor Ink Ag Powder 60.00-90.00 Resin  0.00-15.00 50/50lecithin in terpineol 1.00-6.00 Cu-100 0.50-1.00 SiO₂  0.00-15.00 betaencryptite  0.00-20.00 dispersant 0.00-2.00

TABLE 9 Resin 5 B-98 resin 15.00 g Butyl carbitol 34.00 g Dodecanol51.00 g 100.00 g

TABLE 10 Conductor Fill Paste Ag Powder EG (^(˜)7.6 microns) 55.90 g PASglass 22.30 g Hypermer PS-2 1.20 g EC N-300 0.69 g Butyl carbitol 7.91 gElvacite 2045 1.80 g Terpineol 5.40 g #160 plasticizer 3.60 g Thixotrol1.20 g 100.00 g

In the above conductor fill paste compound, PAS glass is 50% PbO, 40%SiO₂ and 10% Al₂O₃, all by weight. In the composition, the PAS glass isfirst made and ground into a powder before being used in the conductorink.

Although the compositions given in Tables 8 and 9 perform well, theinventors have determined that compositions corresponding to the ranges,defined by percent weight, set forth in Tables 11 and 12, produceconductor ink formulations acceptable for use with the presentinvention.

TABLE 11 GBC Formulation: 1 19 27 27a 31 31a 32 33 34 35 36 36a 37Component: Degussa silver powd. EG 81.74 Degussa silver powd. 87.5978.43 80.54 79.47 81.63 77.43 72.28 65.02 67.71 79.06 79.65 73.26 EG-EDDegussa Cu-100 0.68 0.73 0.65 0.67 0.66 0.68 0.59 0.61 0.55 0.58 0.610.56 0.62 Resin 6 16.35 8.76 10.46 10.74 50/50 wt % lecithin in 1.232.92 5.23 2.68 5.30 2.72 2.94 3.21 3.46 3.60 2.79 2.54 3.25 terpineolBeta eucryptite 5.23 5.37 5.30 5.44 8.74 12.69 17.13 15% B-98 resinsolution 9.27 9.52 10.30 11.22 13.84 14.41 11.17 10.18 13.00 GeltechSiO2 (1u) 13.69 6.37 5.80 9.88 Hypermer PS2 1.27

TABLE 12 Resin 6 Ethyl cellulose N-300 79.51 g Ethyl cellulose N-14 1.60g Butyl carbitol 11.33 g Dodecanol 7.56 g 100.00 g

These conductor inks are screen-printed onto the green tape prior to thebarrier formation process. The inventors have determined that thesepastes withstand the barrier formation processes, particularly the highpressures, without breaking any conductor traces while maintaining goodelectrical conductivity. The fill paste formulation set forth in Table11 matches the sintering characteristics and the coefficient of thermalexpansion of the glass-ceramic formulation described above withreference to Tables 1 to 3.

During the laminating process, the green tape is bonded to the metalcore using a glaze. An exemplary glaze suitable for this purpose isformed from a powder of Glass 1 to bond the green tape formulationdescribed above with reference to Tables 1 to 6, to the titanium metal.This powder is sprayed onto the metal before forming, for example, asix-layer laminate of green tapes. Alternatively, a commerciallyavailable low-melting point lead-based glaze, for example, Homel F-92,may be applied to the surface of the titanium core prior to lamination.The inventors have determined that the application of this glaze reducesoxidation of the titanium and allows good mechanical locking of thetitanium to the glass-ceramic composition in the lamination andco-firing processes.

A suitable bonding glaze may be made by reducing the particle size toless than 2 microns, mixing the glass powder with an organic liquid of aknown amount, applying the mixture to the metal to achieve a knownamount of glass on the metal core, for example 10 mg/inch². The glaze onthe metal is then pre-flowed in a furnace with a peak firing temperatureof approximately 550° C.

An exemplary glaze of the present invention is set forth in Tables13-15.

TABLE 13 Glaze 1 Glass 1 1.25 g Glass 3 3.75 g Glass 4 1.25 gIsopropanol 93.75 g 100.00 g

TABLE 14 Glass 3 - weight percent BaO <30.00 B₂O₃ <10.00 SiO₂ <10.00 CaO<10.00

TABLE 14 Glass 4 - weight percent PbO <75.00 Al₂O₃ <25.00 B₂O₃ <25.00SiO₂ <5.00 ZnO <1.00

The inventors have determined that a laminate made using the green tapeformulation set forth in Table 7 exhibits a good thermal expansioncoefficient match to the titanium metal. The ceramic produced when thegreen tape and metal core are cofired has a dielectric constant ofapproximately 7. It also exhibits rheological properties, when in thegreen state, which enable groove formation suitable for the preparationof barrier ribs. Typical barrier ribs of up to 400 microns in height maybe formed on the green tape, as described below with reference to FIGS.3 and 4A, resulting in barrier ribs having a height of approximately 274microns on the co-fired back plate.

It is well known in display technology that a black background beneaththe red, green and blue phosphors enhances contrast in displayed images.This aspect of display technology may be implemented in a relativelystraightforward manner by adding a black coloring agent, such as cobaltoxide (CoO) into the formulation of the top layer of the tape or byspraying or screen painting a paste of the black coloring agent onto thetop layer of the tape either before or after the co-firing of the panelat step 124 (shown in FIG. 1) but before the application of thephosphors at step 126. In the present invention, a black dielectric inkcan be used to implement the above aspect of display technology. Theblack dielectric ink composition used must exhibit good printability andcompatibility with other materials and processes utilized herein.

An exemplary composition of a black dielectric ink composition of thepresent invention is set forth in Tables 16 and 17.

TABLE 16 Glass 1 3.00-50.00 Glass 2 0.00-15.00 Glass 3 0.00-45.00 Resin0.00-40.00 TiO₂ 0.00-22.00 ZrO₂ 0.00-23.00 Zn stearate 0.00-5.00  50/50lecithin in terpineol 1.00-4.00  50/50 butyl carbito/dodecanol0.00-4.00  black pigment 0.50-7.00 

TABLE 17 Resin 7 B-98 resin 12.00 g #160 plasticizer 3.00 g Butylcarbitol 34.00 g Dodecanol 51.00 g 100.00 g

Although the compositions given in Tables 14 and 15 perform well, it hasbeen shown by the inventors that compositions corresponding to theranges, defined by percent weight, set forth in Tables 18 and 19,produce black dielectric ink formulations acceptable for use with thepresent invention.

TABLE 18 DE Ink Formulation: 123b 183b 149b 206b 229b 235b 229b 229b-r1229b-r2 229b-r3 229b-r4 Component: Glass 1 (3.5-10 um) 48.98 38.10 40.7124.99 21.47 21.80 33.76 40.05 31.77 34.73 19.76 Glass 2 (3.5-4.5 um)12.24 9.52 10.18 6.24 5.62 5.70 8.83 10.47 8.33 9.08 5.17 TiO2 (Cerac1u) 16.49 15.32 7.02 8.32 13.13 21.54 18.72 white spinel (3.5-4.5u) 4.765.09 6.09 ZrO2 (1.5-3.5u) 4.76 14.60 17.26 16.05 7.35 8.72 13.13 22.5719.61 Zn stearate 4.72 1.10 1.12 0.80 1.07 1.00 1.29 0.90 MSR-80(4.5-5.5u) 0.83 Cordierite (3.5-4.5u) 1.37 Black pigment (Cerdec 6.124.76 5.09 4.72 4.40 4.47 4.53 5.37 5.02 6.47 4.50 9585) Resin 6 30.6134.92 34.65 34.71 50/50 lecithin in terpineol 2.04 3.17 3.39 3.15 2.932.98 3.02 3.58 3.34 4.31 3.00 15% B98/S-160 35.54 30.73 31.19 28.35 20%B98/S-160 22.41 20.94 60/40 BC/DD 3.34 229b- 229b- 229b- 229b- 229b-229b- DE Ink Formulation: 229b-r5 229b-r6 229b-r7 229b-r8 229b-r9 r7ar10 r11 r12 r13 r13a Component: Glass 1 (3.5-10 um) 16.78 18.25 21.5321.21 18.25 19.11 21.89 38.59 21.84 21.22 11.43 Glass 2 (3.5-4.5 um)4.51 4.78 5.63 5.55 4.78 5.00 5.73 9.66 5.71 5.55 2.99 TiO2 (Cerac 1u)15.90 17.29 16.53 16.28 17.29 14.67 16.81 7.68 16.77 16.29 8.78 whitespinel (3.5-4.5u) ZrO2 (1.5-3.5u) 16.65 18.11 17.31 17.05 18.11 15.3617.60 7.98 17.56 17.06 9.19 Zn stearate 0.76 0.83 0.84 0.87 0.83 0.981.12 1.00 1.12 1.08 0.58 MSR-80 (4.5-5.5u) Cordierite (3.5-4.5u) Blackpigment (Cerdec 3.82 4.15 4.41 4.35 4.15 3.92 4.49 1.00 4.48 1.08 0.589585) Resin 6 50/50 lecithin in terpineol 2.55 2.77 2.94 2.90 2.77 2.612.99 2.97 2.98 3.29 1.77 15% B98/S-160 39.02 33.81 30.81 31.80 33.8131.13 34.43 20.37 20% B98/S-160 38.36 60/40 BC/DD ESOL-4 29.38 ESOL-5(Elvac) 29.54 F-92 44.29 229b- 229b- DE Ink Formulation: 240b r13b r13c238b 239b 240b 242b 243b 244b 245b 246b Component: Glass 1 (3.5-10 um)22.30 4.04 13.85 21.82 24.15 21.54 20.66 22.10 26.14 24.49 22.59 Glass 2(3.5-4.5 um) 5.83 4.04 3.63 5.71 6.32 5.64 5.41 5.78 6.40 5.91 TiO2(Cerac 1u) 15.34 11.84 10.63 15.34 12.78 14.82 15.24 13.57 16.70 13.9211.96 white spinel (3.5-4.5 u) 3.42 ZrO2 (1.5-3.5u) 15.87 12.40 11.1316.07 13.28 15.33 15.97 14.21 17.49 14.57 12.52 Zn stearate 1.14 0.790.71 1.12 1.13 1.10 1.11 1.13 1.11 1.14 1.16 MSR-80 (4.5-5.5u)Cordierite (3.5-4.5u) Black pigment (Cerdec 4.57 1.19 1.07 4.47 4.504.41 4.44 4.53 4.46 4.56 4.63 9585) Resi n6 50/50 lecithin in terpineol3.05 2.39 2.15 2.98 3.00 2.94 2.96 3.02 2.97 3.04 3.09 15% B98/S-16031.91 27.48 24.67 31.23 31.43 30.83 31.03 31.63 31.13 31.87 32.33 PASglass (sem-com) 3.38 F-92 35.84 21.45 1.94 Geltech SiO2 1.25 1.24 1.71Panel glass powder 10.73 Panel glass 3.6u 4.02 4.10

TABLE 19 Resin 7 B-98 resin 16.00 g #160 plasticizer 4.00 g Butylcarbitol 32.00 g Dodecanol 48.00 g 100.00 g

At step 122 of FIG. 1, after the green tape has been laminated to attachit to the metal core, it is embossed or scribed to form the barrierribs. Although scribing may be used to form the barrier ribs, theembodiments of the invention described below focus primarily onembossing as a means to form the ribs.

FIG. 3 is an isometric drawing of an embossing tool or die which may beused to form barrier ribs in the laminated green tape structure. The dieis a “negative” of the desired barrier structure in the green tape. Thedie shown in FIG. 3 is made by stacking together sheet metal stripswhich alternate in width and thickness. The thickness of the differentstrips determines the die pitch and the difference in the width of thestrips defines the rib height. A die of this type may be fabricated fromany material which can be formed into strips, including stainless ortool steel, plastics or ceramics.

The exemplary die includes alternating thick strips 310 and thin strips312. The thick strips 310 define the channels between the barrier ribsin the finished panel. Thus, the thick strips 310 are wider than thethin strips 312 to allow the barrier ribs to form between the thickstrips. These wider strips desirably have smooth surfaces to help thegreen ceramic material to flow and release easily during the embossingprocess. The edge regions of the wider strips (e.g. the region 320) mayalso have a shape that is modified from rectangular (e.g. rounded) toprovide an edge with good release characteristics and which alsoachieves the desired barrier shape in the green tape after embossing.

The two sets of metal strips having the same length but differentthickness and width may be prepared, for example, using electrodischarge machining (EDM) to achieve the desired smoothness. The stripsare stacked alternately, one strip at a time from each set. The stripsare desirably accurately positioned, for example, using a flat metalplate 314 and/or metal rails 318. The stack is then compressed from itssides to ensure good packing and uniform pitch. The stack may be securedin the compressed dimension by any of several known methods, such aswelding, soldering, gluing or packing into a mandrel 316. Because thethickness tolerance of the metal strips is additive throughout thestack, a compressible metal (e.g. annealed copper) may be used for thethin strips of smaller width. The compressibility of these metal stripscompensates for thickness tolerances.

The wider strips may be machined to modify their edges either before orafter they are stacked to achieve angles that readily release the greentape during the embossing process. The edges may be modified by any of anumber of known techniques such as glass bead blasting, sand blastingand lathe machining.

A die prepared as described above has several advantages over aconventional die, machined from a single piece of metal. It may be madevery large and, thus, accommodate large-screen plasma displays; it mayalso be easily repaired or modified resulting in a longer lifetime thansingle-piece dies.

The die is pressed against the green tape, preferably after the greentape has been laminated to the metallic core in order to form thebarrier ribs. When the laminated tape is embossed, the metal core formsa rigid substrate. The inventors have determined that the presence ofthis rigid substrate significantly enhances perpendicular flow of thegreen ceramic material, allowing it to more readily take the shape ofthe embossing tool. The inventors have also determined that it isadvantageous for the pressing operation to be pulsed, that is to say,repeatedly pressing the die against the laminated green tape atrelatively high pressure, separated by relaxation periods of zeropressure, all at a constant temperature. The application of highpressure forces the green tape into the voids between the thick bars ofthe die to form the barrier ribs. The relaxation periods cause theparticles of the green tape to move apart, allowing the organic materialto flow back between them, decreasing the viscosity of the green tape.Using this technique, the maximum pressure applied to the laminatedgreen tape structure is decreased which decreases deformation and wearon the die. Barrier rib aspect ratios as high as 10 to 1 may be readilyachieved using the embossing technique described above.

It is desirable, using this technique, for there to be a completerelease of the tape from the die. One method by which this may beachieved is to place elastic spacers (not shown) under the die butoutside of the embossed area. These spacers are compressed by theembossing pressures and provide a lifting force on the tool when thepressure is released. The inventors have determined that thepressurization of the elastic spacers prior to pressing the tool intothe green ceramic also improves pressure distribution in the assemblyand, thus, makes the height of the embossed barriers more uniform overthe entire embossed area. It is contemplated that springs (not shown)may be used instead of elastic spacers.

FIG. 4A shows an exemplary barrier rib structure that may be achievedusing the embossing technique described above. In the design of aparticular panel, it may be desirable to achieve a barrier rib shapesuch as that shown in FIG. 4C. This structure may not be formed by asimple embossing operation because the barrier ribs are wider at the topthan at the bottom and, thus, do not permit an embossing die to bewithdrawn. A structure such as that shown in FIG. 4C may be made,however, by processing an embossed structure using a laminating tool. Asa first step, the green tape is embossed, as shown in FIG. 4B, toachieve a barrier height that is taller than desired. Next, a laminatingtool presses against the tops of the green ceramic barrier ribs with acontrolled pressure, causing them to bulge as shown in FIG. 4C. Thepressure exerted by the laminating tool depends on the viscosity of theembossed green tape and on the amount of bulge that is desired in thetips of the ribs.

FIG. 5 is a flow-chart diagram which illustrates the formation of aflat-panel display device according to the present invention. Thisflow-chart diagram is more detailed than the one shown in FIG. 1. Inparticular, the metal preparation step 111, in which a bonding glaze isapplied to the titanium core, and tape preparation step 113, in whichthe tape blanks are cut, are explicitly shown. In addition, step 120 isdivided into separate steps 120′, in which the titanium core and thetape blanks are stacked, and step 121 in which the tape blanks arelaminated to each other and bonded to the titanium core in a singlestep. Step 122′ is also changed from FIG. 1 in that it recites that thebarriers are formed using pulses of pressure. Finally, FIG. 5 differsfrom FIG. 1 by including the front panel glass 510 and the step 512 bywhich the front panel is attached to the back panel.

FIG. 6A is a cut-away side plan view taken along a line taken along arow of the display which illustrates the structure of a plasma displaypanel according to the subject invention. The panel includes a backpanel 610 which includes a titanium core 612, and a laminated andembossed ceramic structure 614 which forms the barrier ribs for thedisplay as well as the substrate on which the electronics 626 thatcontrol the display are mounted. Embedded in the ceramic structure 614are the pixel electrodes 620. Above each electrode, on the surface ofthe embossed ceramic back panel is the phosphor 618 which is excited inorder to emit colored light (red, green or blue). Above the back panel610 is the front panel 622. The front panel is bonded to the back panelby a frit seal 624.

Although the barriers for the display panel have been described as beingformed on the back panel during the formation of the back panel, theycan alternatively be formed on the glass front panel. The fabrication ofthese barriers on the front glass sheet can be accomplished in thefollowing ways:

Embossing or scribing a thick film of dielectric, deposited on top ofthe sheet glass in the form of a printable paste. A thick film of adielectric, that is made by mixing frit glass powder materials withorganics and is compatible with the sheet glass thermal and chemicalproperties. The dielectric may be deposited as a thick film by thickfilm deposition techniques, such as screen printing, after initialprocessing of glass, such as cleaning and/or electrode deposition. Afterthe dielectric deposition, the thick film is embossed by means of a diethat has the reverse of the desired rib structure such that theembossing/stamping of the thick film dielectric using this die gives thedesired rib structure. Alternatively, the barrier rib structure can beachieved by scribing the thick film dielectric by means of a hard tool,capable or removing the dielectric material, and hence providing thechannel-like ribbed structure. The embossing procedure can be utilizedto make either the AC or the DC plasma display structure and is doneprior to sintering/firing of the thick film dielectric. The scribingprocess, which may only be used to form an AC plasma display structure,however, may be done either before (preferably) or after the firing ofthe dielectric.

A frit material that is compatible with the sheet glass thermal andchemical properties can be cast in the form of a single or multiplethick film tapes that can be embossed, using a die as described above,to form the ribbed structure. The embossing process can occur eitherafter the tape stack (laminate) has been culminated to pre-processed(cleaned and with or without electrodes deposited) sheet glass or can beembossed before and then culminated on top of the sheet glass. Theembossed pattern can then be fired/sintered. In the case where the tapestack is first laminated on top of the pre-processed sheet glass, theribbed structure can be formed by scribing with a hard tool that iscapable of removing the laminated tape material cleanly and efficiently.

Embossing or scribing the sheet glass while hot and in the sheet glassmanufacturing line itself. Either an embossing die or scribing tool canbe used in the sheet glass manufacturing plant on-site making use of theease-of-forming process of the glass while it is hot.

The front panel is bonded to the back panel using a frit material, whichis applied to the periphery of one or both panels. The panels aresupported together and heated to a sufficiently high temperature to meltthe frit material. The panels are then cooled, whereupon the fritsolidifies and forms a gas-tight seal. The frit sealing temperature istypically in the range of 350° to 450° C. While it is desirable toclosely match the TCE's of the front and back panels to ensure that thefrit seal does not break during cooling or operation of the display, itmay be difficult to obtain an exact match.

To compensate for minor differences in the thermal expansioncoefficients when forming the seal between the back panel and the frontpanel, the panels can be heated to slightly different temperaturesduring the bonding process. If the frit seal temperature is T_(s), and,if between room temperature and T_(s) the glass has an average TCE(TCE_(g)) that is ΔTCE lower than that of the back panel, TCE_(b), andif the glass is heated ΔT_(s)=T_(s)·ΔTCE/TCE_(g) hotter than the backpanel, then the total contraction of the glass front pane,(T_(s)+ΔT_(s))·(TCE_(b)−ΔTCE), will be the same, to 2^(nd) order, asthat of the back panel (T_(S)·TCE_(b)). For example, if soda-lime glassis used for the front panel with TCE_(g)=8.5×10⁻⁶/° C., and if theTCE_(b) of the back panel is ΔTCE=0.1×10⁻⁶/° C. greater than that of thefront panel, and frit sealing is accomplished at 450° C., then thesystem can be compensated by heating the glass front panel 5.3° C.hotter than the back panel during frit sealing.

Various methods may be used to implement this temperature difference.For example, heating sources above and below the panels in the furnacemay be heated to different temperatures establishing a temperaturegradient between the front panel and back panel. Alternatively, thisprocess may be implemented by variation of cooling provisions for thetop and bottom of the sealed assembly after it is removed from thefurnace.

Moreover, this approach may be used more generally, to compensate fordifference in temperature dependence between the front and back panels.In a belt furnace, the cooling zones may be programmed to producespecific temperature differences between the front and back panels as afunction of the panel temperature to provide compensation for TCEdifferences as the assembly cools.

An alternative approach to compensate for small differences in TCEbetween the metal/ceramic back panel and the glass front panel is toformulate a ceramic composition which has a TCE that is slightly greaterthan, or slightly less than that of the metal core. The compositeceramic-metal system has a TCE which is intermediate between the twoTCEs. The TCE of the ceramic may be adjusted within a small range sothat the intermediate value matches the TCE of the front panel. Inaddition, the ceramic material may be bonded both to the front and backof the metal core to prevent the ceramic-metal assembly from warpingwhen it is cofired due do the difference between the respective TCEs.

Based on the desired characteristics set forth above for formulation,composition and fabrication of a high-resolution LTCC-M plasma displayback panel, the inventors have identified a formulation representing anexemplary embodiment of the present invention. The formulation definedby the combination of green tape, conductor ink, glaze, and blackdielectric ink described in Tables 1, 8, 10, and 13, respectively, canbe used to fabricate defect-free embossed and fired LTCC-M back panelsof the present invention.

While the invention has been described in terms of an exemplaryembodiment, it is contemplated that it may be practiced as describedabove with modifications within the scope of the appended claims.

What is claimed:
 1. A composition of materials to fabricate a highresolution, low-temperature cofired ceramic-on-metal plasma display backpanel, the panel comprising a metal core having a predetermined thermalcoefficient of expansion, the composition comprising a glass materialformulation comprising a first glass and a second glass; wherein thethermal coefficient of expansion of the formulation after firing closelymatches that of said metal core; wherein, said first glass includes ZnO,MgO, B₂O₂, and Si₂, and has a formulation, by weight percent, of: ZnOgreater than 0 to less than and equal to 30 MgO greater than 0 to lessthan and equal to 25 B₂O₃ greater than 0 to less than and equal to 20SiO₂ greater than 0 to less than and equal to 25;

said second glass has a formulation defined by weight percent as: BaO65.20-87.00 B₂O₃  2.60-16.10 SiO₂  9.10-23.00; and

said glass material has a formulation defined by weight percent as:first glass 89.1-65.0 second glass  10.9-35.0.